module-inspector/Editor/Drawers/SerializableReferenceToAttributeDrawer.cs

using System;
using System.Reflection;
using System.Runtime.Serialization;
using Module.Inspector.Editor.Utilities;
using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;

namespace Module.Inspector.Editor
{
    [CustomPropertyDrawer(typeof(SerializableReferenceToAttribute))]
    internal sealed class SerializableReferenceToAttributeDrawer : DrawerPropertyDrawer
    {
        private enum ESupportType
        {
            Supported,
            SpecializeUnityEngineObject,
            NonSerialized,
            GenericType,
            MissingSerializeReference,
            NotManagedReference
        }

        public override bool Draw(Rect position, DrawerPropertyAttribute attribute, SerializedProperty property, GUIContent label, EditorPropertyUtility.Result result)
        {
            if (IsSupported(property) != ESupportType.Supported)
                return false;

            var att = (SerializableReferenceToAttribute)attribute;
            Type type = att.useType ? att.type : property.GetValueType();
            string[] arrStrings = EditorTypeUtility.GetAssignableFromAsStrings(type);
            GUIContent[] arrGui = EditorTypeUtility.GetAssignableFromAsGUI(type);

            EditorGUI.BeginChangeCheck();
            EditorGUI.BeginProperty(position, label, property);
            {
                var rect = new Rect(position)
                {
                    height = EditorGUIUtility.singleLineHeight
                };

                int index = Array.IndexOf(arrStrings, ConvertUnityToCSharp(property.managedReferenceFullTypename));
                int newIndex = EditorGUI.Popup(rect, label, index, arrGui);

                if (newIndex != -1 && index != newIndex)
                {
                    Type newType = EditorTypeUtility.GetType(arrStrings[newIndex]);

                    if (newType != null && IsSupported(newType))
                        property.managedReferenceValue = FormatterServices.GetUninitializedObject(newType);
                    else
                        EditorUtility.DisplayDialog("Error", "Failed to set managed reference to selected type.\n\nType must be:\nSerializable\nNot abstract\nNon-generic\nNot a specialization of UnityEngine.Object", "Ok");
                }

                EditorGUI.PropertyField(position, property, true);
            }
            EditorGUI.EndProperty();
            bool hasChanged = EditorGUI.EndChangeCheck();
            
            if (hasChanged)
                property.serializedObject.ApplyModifiedProperties();
            
            return true;
        }
        
        public override string GetErrorMessage(SerializedProperty property)
        {
            ESupportType supportType = IsSupported(property);

            switch (supportType)
            {
                case ESupportType.SpecializeUnityEngineObject:
                    return "Must not be a specialization of UnityEngine.Object";
                case ESupportType.NonSerialized:
                    return "Must be serializable";
                case ESupportType.GenericType:
                    return "Must not be a generic type";
                case ESupportType.MissingSerializeReference:
                    return "Missing [SerializeReference]";
                case ESupportType.NotManagedReference:
                    return "Must be a managed reference type";
            }
            
            return "Unknown error";
        }
        
        private static ESupportType IsSupported(SerializedProperty property)
        {
            if (property.propertyType != SerializedPropertyType.ManagedReference)
                return ESupportType.NotManagedReference;
            
            Type type = property.GetValueType();
            
            if (!type.IsSerializable && !(type.IsInterface || type.IsAbstract))
                return ESupportType.NonSerialized;
            if (typeof(Object).IsAssignableFrom(type))
                return ESupportType.SpecializeUnityEngineObject;
            if (type.IsGenericType)
                return ESupportType.GenericType;

            FieldInfo fi = property.GetFieldInfo();

            if (fi != null && fi.GetCustomAttribute<SerializeReference>() == null)
                return ESupportType.MissingSerializeReference;
            
            return ESupportType.Supported;
        }
        
        private static bool IsSupported(Type type)
        {
            if (!type.IsSerializable)
                return false;
            if (typeof(Object).IsAssignableFrom(type))
                return false;
            if (type.IsGenericType)
                return false;

            return true;
        }

        private static string ConvertUnityToCSharp(string unityNaming)
        {
            int index = unityNaming.LastIndexOf(' ');

            if (index != -1)
                unityNaming = unityNaming.Substring(index);

            unityNaming = unityNaming.Replace('/', '+');
            return unityNaming.Trim();
        }
    }
}